Open Access Library Journal Radiometric Study in the Itaboraí Region, Brazil Hans Schmidt Santos1, Luíza Viênna Rangel Peixoto2 1Department of Research and Extension, Faculdade Salesiana Maria Auxiliadora, Macaé, Brasil 2Chemical Engineering College, Faculdade Salesiana Maria Auxiliadora, Macaé, Brasil Email: [email protected], [email protected] Received 1 July 2014; revised 25 August 2014; accepted 28 September 2014 Copyright © 2014 by authors and OALib. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ Abstract This current paper discusses an application of the aerial radiometric method in the Itaboraí Re- gion, located in Rio de Janeiro State, Brazil. Radiometric data acquired by aircraft registered in counts per second of potassium, thorium and uranium channels were processed generating radi- ometric maps of the entire region. The generated maps provide important information regarding the superficial geology. At last, all the radiometric information gained is correlated with geological and topographical knowledge of the Itaboraí Region. Keywords Aerial Radiometry, Potassium, Uranium and Thorium, Itaboraí Region Subject Areas: Chemical Engineering & Technology, Geochemistry, Geology, Geophysics 1. Introduction Several applications are derived from the natural radioactivity of the chemical elements. Among the radioactive emissions, the gamma emission being a detectable electromagnetic wave at large distances has a special applica- tion in radiometric method. In this method, large areas can be investigated as for its shallow geological heritage, by detecting the radioactivity emanated from the constituent rocks from the soil and underground. In the Itaboraí Region, it stood out the São José do Itaboraí Basin which was the scene of exploitations of limestone for many decades. After finding large amount of fossil, it is today an important paleontological park in the region. Aerora- diometric data referred to Itaboraí covering the basin are processed and the main results can be used for better understanding of the regional geology. Moreover, it can serve as the basis for future geological, environmental and geochemical studies in the region. 2. Characteristics of the Studied Area The Itaboraí Region is located in the metropolitan region of Rio de Janeiro state, in an area of coastal lowland with How to cite this paper: Santos, H.S. and Peixoto, L.V.R. (2014) Radiometric Study in the Itaboraí Region, Brazil. Open Access Library Journal, 1: e921. http://dx.doi.org/10.4236/oalib.1100921 H. S. Santos, L. V. R. Peixoto margins in the Guanabara Bay (Figure 1). The county covers an area of 430.4 km2. The neighboring counties are Guapimirim, Tanguá, Cachoeiras de Macacu, Maricá and São Gonçalo [1]. The region of Itaboraí presents a varied relief and low altitudes. The topography of the region is shown in Figure 2. Its highest elevations are found in the Barbosão Sierra located to the east on the border with Tanguá County, the Lagarto Sierra and the Cassorotiba do Sul Sierra, bordering the Maricá County. In other parts of the county pre- dominate plains which have rivers that converge to the Guanabara Bay. In the plains and mountains, there is a soft, wavy relief, with small hills which few times exceed 50 meters of altitude. The vegetation of the county is mainly composed of pastures, hillside forest, swamps and marshes (Figure 3) [2]. The oldest Brazilian paleontological site is located in the sedimentary basin of São José do Itaboraí in Itaboraí. The São José do Itaboraí Basin is considered one of the smallest known sedimentary basins in Brazil. This basin stands out because of its diverse lithology and its rich fossil content [3]. The sediments of the basin include clasts of angular fragments of quartz and feldspar, and pieces of rocks, in pelitic carbonate matrix, containing abundance of gastropod fossils. The crystalline basement of the basin consists of igneous and metamorphic rocks such as gneisses, migmatites and granites [4]. The geology of the region of the county of Itaboraí is shown on Figure 4. Figure 1. Localization of Itaboraí County in Rio de Janeiro, Brazil [1]. OALibJ | DOI:10.4236/oalib.1100921 2 September 2014 | Volume 1 | e921 H. S. Santos, L. V. R. Peixoto Figure 2. Topographic map of Itaboraí and surroundings. Figure 3. Typical mangroves of the Itaboraí Region [2]. The whole Itaboraí County is covered by aeroradiometric surveys made by a Bandeirante aircraft with sampling intervals of 100 m, 1 km spaced profiles and measurement lines in the N-S direction. The gamma spectrometer used in the survey was the Geometrics GR-800A with sodium iodide crystals activated by thallium and volume of 3072 cubic inches. These collected data were used in this work. OALibJ | DOI:10.4236/oalib.1100921 3 September 2014 | Volume 1 | e921 H. S. Santos, L. V. R. Peixoto (a) (b) Figure 4. (a) Geolologic map of the Itaboraí County and surroundings [5]. (b) Legend of the Itaboraí gologic map [5]. OALibJ | DOI:10.4236/oalib.1100921 4 September 2014 | Volume 1 | e921 H. S. Santos, L. V. R. Peixoto 3. Methods The radiometric method consists in the detection of nuclear emissions from rocks which contain radioactive minerals. In case, it is detected gamma radiation (electromagnetic wave) using a scintillation counter or Geiger counter. These radiometric instruments were first developed for the detection of uranium, but soon appeared other important applications. The method is widely used for geological mapping, being useful to identify lithology by the radioactive contents [6]. 3.1. Gamma Radiation Sources Gamma radiation has a discrete energy contained in a photon that is characteristic of the emitting material. This is the basis of the gamma spectrometry: by measuring the energy of the photons, one can determine the source of radiation [7]. Potassium (40K), Uranium (238U, 235U and decay series elements) and Thorium (232Th and decay series elements) are the only radioisotopes which emit gamma radiation with intensity large enough to be used in the mapping of gamma radiation. The detection is performed through specific windows (Figure 5) [8]. All radiation that is not derived from geological sources is called background radiation and it needs to be re- moved in data processing. There are three major sources of background radiation: atmospheric radiation, cosmic radiation, and the instrumental radiation. The largest source of background radiation is atmospheric radon (222Rn) and its decay series elements. This undesired effect occurs because the 222Rn can get out of the rocks and accu- mulate in the atmosphere. Its decay series elements (214Bi e 214Pb) can be added to aerosols and dust. But the in- strumental background radiation is related to radiation arising from traces of K, U and Th in the detectors and neighboring equipment, aircraft or vehicle. This background radiation is taken as constant [7]. Figure 5. Typical power windows of Uranium, Thorium and Potassium detection [8]. OALibJ | DOI:10.4236/oalib.1100921 5 September 2014 | Volume 1 | e921 H. S. Santos, L. V. R. Peixoto 3.2. Geochemistry of Radionuclides Potassium (K), Uranium (U) and Thorium (Th) are primarily responsible for the natural radioactivity of the rocks. They are lithophile and are mainly concentrated in acid igneous rocks [9]. Potassium (40K) is the most abundant of the three natural radionuclides. The main sources are the potassium are feldspars, muscovites and biotites. The Uranium (238U) and Thorium (232Th) can be found in small amounts in minerals such as quartz, feldspar and higher concentrations of minerals as the zircon, epidote, apatite and monazite [9]. In igneous rocks, radioactivity can vary greatly due to the chemical, mineralogical, petrographic and structural features. Table 1 summarizes the major radioactive minerals [8]. 3.3. Acquisition and Corrections of Aerial Radiometric Data The gamma radiometric surveys can be terrestrial or aerial. In the terrestrial surveys, the spectrometer is posi- tioned on the ground or on the surface which we want to perform the measurement. In airborne surveys, the equipment stays in the interior of an aircraft which is usually a plane. The basic difference between these methods is the represented area. The flying height and the spacing between flight lines determines the resolution of the survey. The spacing of flight lines are normally about 1 - 2 km on a measurement density on each line around 70 - 100 m [10]. In the present study, the gamma radiometric surveys was aerial mode and the unit used to measure the radiation activity was counts per second (cps). Some corrections and tests need to be applied after the acquisition of aero radiometric data before proceeding to the interpretation of the results. The Low Level test performed for correction of radiometric data is also known as repeatability line. In this case, a measurement ramp needs to be created, usually in an airport runway where the aircraft flies in typical heights of exploration that are around 100 m, then performing collections for around 2 minutes or 10 km measurements. Because the test is always performed on the same ramp, it generates a standard of radiometric measurements for the diurnal variation correction. The High Level test is also called calibration test. It is kept a direction of flight to collect measurements for about 3 minutes at a height of approximately 800 m. In this case, it is not necessary that the ramp measure is always the same because we believe that the data in soil does not interfere with the measurements. The Background test is repeated daily. In this test, environment radiometric measurements are performed. This interference is also known as background measures. In addition to these cor- rections, it may be cited the Compton Effect correction in which the electromagnetic wave undergoes a scattering, increasing its wavelength and reducing the photon energy [11]. 4. Processing and Results In the first step, data from an area spanning Itaboraí (black contour) and also the vicinity of São José de Itaboraí Basin (brown contour) were processed to provide a regional study.